It is known in the art to provide a liquid level sensor comprising a non-magnetic guide tube vertically arranged in the liquid whose level is to be sensed, with a plurality of reed switches disposed spaced apart in the tube to be actuated by a magnet carried by a float guided on the tube and rising and falling with the liquid level. The reed switches are connected by electric circuits to signal various liquid levels, and may serve to energize pumps, alarms, indicators and the like. The following United States Patents are representative of such teaching: U.S. Pat. Nos. 3,198,902; 3,200,645; 3,646,293; 3,678,750; 3,685,357; 3,788,340; 3,826,139.
To obtain a differential switch using the construction of the majority of these patents requires two switches, one at each extremity of travel, with one or more magnets activating the switches. U.S. Pat. No. 3,198,902 attempts to remedy this by using two magnets and a switch which is activated to predetermined conditions when the switch is swept by opposite poles. However, the construction used in this patent is only correctly activated by passage of the magnetic fields through a small sensing area.